1. Field of the Invention
The present invention relates generally to apparatus and methods for controllably dispensing a selected quantity of a fluid from a source thereof to a chosen receptacle therefor. More particularly, the present invention relates to a method, and to particular apparatus for practicing the method, to provide a medical diagnostic analyzer or incubator with a precision fluid dispensing system including a precisely regulated positive pressure source the positive pressure from which is applied to each of a plurality of source fluids which are held in respective closed containers for causing the source fluids to be discharged to the chosen receptacle under control of respective time duration modulated dispensing valves.
2. Discussion of the Related Technology
Analyzers and incubators which are used in the medical field for analysis of chemical and biological constituents of specimens, such as blood samples and other bodily fluids and tissues, require the addition of a variety of precisely-measured fluid reagents to the specimens and incubation cultures. Previously, these analyzers and incubators have included a variety of positive-displacement fluid pumps which directly metered the reagents to the specimens or incubation medium. These positive displacement metering pumps were complex and expensive, as well as requiring considerable maintenance. Sometimes, these pumps could be trouble-prone as well, resulting in shutdown of an analyzer or incubator with consequent loss of test results.
Another conventional fluid dispensing expedient which has been employed by some analyzers or incubators has been the use of a regulated air pressure source including an air pump and a spring-biased type of pressure regulator. The regulated air pressure from this source was supplied to reagent fluid which was held in a closed container, and discharge of the reagent was controlled by a normally-closed, time-duration-modulated dispensing valve. The quantity of reagent dispensed in such a system is a function of the applied pressure and the interval during which the dispensing valve is held open.
However, this conventional fluid dispensing system also is prone to a multitude of difficulties. Because the volume of the specimen or incubation containers, which are the receptacles for the reagents, is ordinarily very small (perhaps on the order of a few milliliters or smaller), precise metering of the reagents is necessary. As mentioned, with the time duration controlled dispensing valves, the quantity of reagent dispensed varies with the applied dispensing pressure. However, this pressure may vary from day to day and from moment to moment during operation of an analyzer or incubator.
For example, the spring-biased air pressure regulator may not be capable of precisely controlling the air pressure level which is applied to the reagent fluids over a period of time and under all operating conditions. That is, the spring bias of the air pressure regulator may decline as the regulator ages and the spring loses pre load. This gradual loss of dispensing air pressure level requires periodic recalibration of the air pressure regulator, with the quantity of dispensed reagents being progressively more uncertain with the passage of time after each calibration. Further, the pressure regulator may be temperature responsive so that the applied air pressure varies with changes in the ambient temperature. This temperature change sensitivity of the dispensing air pressure level can cause the quantity of reagents to vary from hour to hour within a single day.
Also, if there is a small air leak at one of the reagent containers, this reagent container, or all of the reagent containers, may not be provided with the required level of regulated air pressure so that dispensed quantities of this reagent or all of the reagents are short and test results are in doubt. In other words, the conventional spring-biased pressure regulator system may be sensitive to air flow volume so that the required level of air pressure is not provided under certain conditions of air flow. The inherent droop of a spring-biased air pressure regulator system with increasing air flow rate may even result in the air pressure applied to the reagents being less than required simply because of the rate at which reagents are dispensed from the system during a time of heavy demand for the reagents.
Still further, some of the reagents themselves are caustic, acidic or corrosive so that the environment in which the air pressure regulator is used is a difficult one in which to maintain a precise air pressure control with such conventional air pressure regulators. As may easily be appreciated in view of the discussion above describing the tenuous control over air pressure which is provided by a conventional regulator under good operating conditions, if the regulator is subjected to corrosion or other physical deterioration, the regulated pressure level certainly cannot be precisely maintained. All of these error sources tend to be additive so that conventional regulated air pressure fluid reagent dispensing systems have been somewhat troublesome and lacking in reliability.